A storage cell is said to be "industrial" when it is of large capacity, i.e. greater than 10 Ah, and generally of capacity lying in the range 50 Ah to 200 Ah. Such cells generally have a container that is prismatic in shape, being made of a plastics material and containing electrodes that are plane. The parallelepiped shape of such cells and the nature of their cases do not enable them to withstand significant excess pressure, and the maximum internal pressure they can withstand is of the order of 1 bar to 2 bars.
Industrial storage cells of the type that is to said to be "maintenance-free" or of the type that is said to be "sealed" have the advantage of not requiring the aqueous electrolyte to be topped up at any time during use, unlike industrial storage cells of the "conventional open" type or of the "reduced-maintenance open" type. Batteries of such cells are used in particular for electric traction in vehicles. In this application, the required lifetime is about 1500 charge/discharge cycles over a period estimated at 10 years. Such a long lifetime cannot be achieved without enforcing strict limitations on overcharging. In addition, "fleet" type use of such vehicles means that it is necessary to be able to perform recharging in a short length of time, for example it should be possible to recharge 40% of nominal capacity in less than 15 minutes, while also guaranteeing that there will be no overcharging.
While an alkaline-electrolyte storage cell is being charged, the electrochemically active materials of its positive and negative electrodes are subjected respectively to oxidation and to reduction. These reactions take place without any gas being given off. Once the positive active material has been fully oxidized, the cell enters into overcharging. Abundant release of gaseous oxygen then appears on the positive electrode.
Under rapid charging conditions, the oxygen that is released has no time in which to be reduced (or recombined) at the negative electrode. This gives rise simultaneously to an increase in the temperature and in the internal pressure of the storage cell. "Maintenance-free" or "sealed" type industrial storage cells have a maximum operating pressure of about 2 bars. Any increase in internal pressure leads to a valve opening, and thus to a loss of electrolyte, thereby reducing the lifetime of the battery by drying out. In addition, significant heating occurs which is also prejudicial to the lifetime of the storage cell.
It is therefore necessary to have a method that makes it possible to recharge an alkaline-electrolyte industrial battery, in particular of the "maintenance-free" or "sealed" type, and to do so in a short period of time while nevertheless avoiding overcharging: charging must be stopped no later than the battery reaching a charge percentage of 100%. The method must also be applicable regardless of the initial state of charge of the battery and regardless of its temperature.
End-of-charge criteria have already been proposed such as monitoring the increase in temperature (+.DELTA..theta.) of a storage cell, the rate at which its temperature varies (d.theta./dt), or indeed the increase in temperature of the cell compared with expected heating conditions.
These criteria are suitable for so-called "portable" storage cells which are of small size and small capacity (up to 10 Ah). They have a container which is generally in the form of a metal cylinder. Portable storage cells have low thermal inertia: they heat up relatively little, but they are highly sensitive to variations in outside temperature. This can be taken into account by effecting a correction for ambient temperature. These criteria can also be used under slow charging conditions with industrial storage cells of small capacity, providing thermal inertia is minimized.
Such a correction has no effect on an industrial storage cell having capacity that is significantly greater than that of portable storage cells, in particular of capacity greater than 50 Ah. Industrial cells heat up significantly while the nature of the case (plastics material) is not suitable for easily dumping the heat generated during charging. The thermal inertia of such cells makes it impossible to stop charging on the basis of cell temperature.
In addition, criteria using temperature are difficult to apply to the Ni-MH couple because of its continuous increase in temperature throughout the duration of charging.
Furthermore, the voltage drop criterion (-AV) commonly used for the Ni-Cd couple cannot be applied in the present case since that criterion corresponds to overcharging that has already begun. In addition, a criterion based on voltage is not very reliable because of the small voltage signal delivered by the Ni-MH couple at the end of charging (0 mV to -5 mV).
For a storage cell of high capacity, and in particular an Ni-MH cell, rapid rate charging governed by the above-mentioned previously-known criteria leads either to end of charging being detected prematurely, so that charging is insufficient, or else to end of charging not being detected, in which case the cell is destroyed.